1. Field of the Invention
The invention relates generally to screen printing of thin and thick film circuitry and more particularly to a stretch resistant screen printing arrangement.
2. Description of Related Art
Conventional screen printing apparatus include a wire mesh layer which is coated with photosensitive emulsion. In the wire mesh, rectangular or circular openings are formed by removing selected portions of the emulsion in a patterned opening portion of the mesh layer through which screen printing paste passes during a screen printing process. The mesh is typically attached to an aluminum frame. A paste of metallic organic or dielectric material is pressed through the pattern openings onto a substrate thereunder by a squeegee, which is moved over the screen.
During the screen printing process, however, the screen may be stretched as a result of the downward and lateral pressure exerted on the screen by the squeegee. Such stretching of the screen can cause distortion in the desired pattern being printed, such as the pattern for conductor lines. Conductor lines may be incomplete or spacing between adjacent conductor lines may be inadequate. Attempts have been made to deal with the screen stretching problem. For example, larger diameter wires have been used for the screen mesh which gives some strength to the screen, but limits the degree to which fine conductor lines can be achieved Screen printing geometric patterns have been adjusted for "light touch" squeegee application to minimize screen stretching. However, some stretching still occurs which limits dimensional accuracy of the printed pattern. Alternatively, nickel foil has been electroplated to a stainless steel mesh. Although this technique limits screen stretch, the pattern art work of the screen must be modified. The expense of fabricating the screen therefore makes this approach an undesirable alternative.
Today electronic circuitry is being made smaller and smaller to meet size and weight requirements desired by both military and commercial customers. Furthermore, as such circuits are used in remote environments reliability is demanded. Accordingly, precise silk screening of nondistorted metallic or dielectric patterns such as conductor lines is desired by the industry. Presently, no screen printing arrangement has been realized to deal with the aforedescribed problems effectively and economically.